A process for evaluating an error in the aircraft positioning as generated by an on-board positioning system is disclosed. The aircraft positioning as generated by the positioning system is recorded through radar means, the presence of at least one group of targets arranged on the ground is detected through said radar means, the position of such a group of targets is measured through said radar means with respect to the aircraft positioning, and the error in the aircraft positioning is evaluated, comparing with at least one threshold value the value of at least one reference magnitude being characteristic of the difference existing between the measured position and a known position of the group of targets with respect to the aircraft positioning.